Greater levels of physical activity relate to obesity prevention and reduced risk of some cancers, heart disease and diabetes. A growing number of correlational studies demonstrate how physical activity and obesity relate to active living community design features, such as walkable neighborhoods and active transportation opportunities including light rail stops, bike paths, and multi-use trails. Yet few prospective studies exist to document health effects of natural interventions in neighborhoods receiving physical changes supportive of healthy physical activity. This study takes advantage of a rare opportunity to conduct a quasi-experimental study in a neighborhood before and after a particularly rich natural intervention creates a Complete Street with a new rail line, a bike path, and a multi-use trail. Complete Streets are increasingly popular designs intended to support transportation by modes other than cars. We study the new Complete Streets both by direct observation and through the experiences of local residents. First, trained observers will count users of the Complete Street both pre- and post-construction of the Complete Street elements, with nearby streets serving as controls. Second, we compare residents living close to (intervention group) and far from (control group) the Complete Streets intervention before and after construction. Residents will be measured for BMI (height, weight), wear accelerometers and GPS units for one- week pre- and post- Complete Streets construction and complete surveys for socioeconomic, attitudinal and behavioral data. We will also assess both macro environmental walkability for large areas of the neighborhood and micro environmental walkability for the routes to and along the Complete Street and nearby streets using secondary data and block by block audits by trained raters. These methods enable us to detail where and how much physical activity residents obtain and whether social, behavioral, and health-related changes are associated with the Complete Streets intervention. We examine effects at Time 1 (pre-construction), Time 2 (post-construction) and Time 3 (follow-up). We hypothesize several effects, net of controls: a) The Complete Street intervention relates to an increase in transit riders, cyclists, and pedestrians, compared to control streets from Time 1 to Times 2 and 3, according to structured observations. b) compared to living far, living close to the Complete Street intervention relates to bigger Time 1 to Time 2 increases in active use of the Complete Streets (e.g., walking, bicycling, as measured by GPS, accelerometry and self-report). c) Resident-perceived walkability and rated walkability relates to greater use of the Complete Street area, both cross sectionally and from Time 1 to Time 2 (according to surveys, environmental audits, and GIS indicators of walkability). A secondary analysis tests whether perceived crime dampens female use. d) Users of Complete Streets will demonstrate healthier changes in Moderate to Vigorous Physical Activity (MVPA from Time 1 to 2), BMI and obesity status (Times 1, 2, & 3) compared to non-users.